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ALEXANDER  VON HTTMBOLDT.                185
them their permanent or occasional heat (sometimes
from ninety-five to ninety-seven degrees), that the
hottest are always the purest, and that the greater
heat of the springs must be occasioned by their rising
from a deeper, i.e., hotter source.
Humboldt has given some interesting explanations-
of the slime which is ejected from some portions of
the earth, founded on his investigation of the American
and Caspian mud volcanos. He considers these erup-
tions, which are not caused by earthquakes or by fire.,
as a link of the chain of phenomena by which the
earth's centre acts on its surface, as something between
fire volcanos and hot springs ; they are the organs of
an uninterrupted but weaker action of the planet, by
which a communication between the centre and the
surface had formerly existed, but which have been
choked up, and where the cold slime now rises from
an inconsiderable depth, while the fire volcanos are
yet in direct communication with the glowing centre.
Humboldt has made considerable investigations into
their origin; he considers them vaulted elevations of
the earth's surface, raised by elastic vapours, which
have broken through and separated the earth/s strata.
This produced a basin,4* or kettle-shaped hollow, in
whose centre a crater and cone formed itself. Wheti
the connexion between this opening and the inner
earth is stopped, the volcano is extinct. In a similar
manner volcanos have risen -from the bottom of the
sea, and have become inhabitable islands.
We have yet to mention another phenomenon in-
vestigated by Humboldt, namely, the volcanic storms,
so named by himself. These are formed by the hot
vapours which rise in the air during the eruption of a
volcano, and which, in cooling, form a cloud which
surrounds the fire-pillar often many thousand feet
high, and from which lightning breaks forth and
thunders roll ; these are caused by the sudden con-
densation of valour into clouds, which excites the
violent electric discharge.
Being unwilling to enter into the details of Hum-